As wells are drilled to greater lengths and depths, it becomes necessary to provide a liner (“casing”) to avoid undesirable fluid inflows or outflows and to prevent borehole collapse. The annular space between the borehole wall and the liner is usually filled with cement to reinforce structural integrity and to prevent fluid flows along the outside of the liner. If such fluid flows are not prevented, there is a loss of zonal isolation. Fluids from high-pressured formations can enter the borehole and travel along the outside of the casing to invade lower-pressured formations, or possibly to exit the borehole in a mixture that dilutes the desired production fluid. Results may include contamination of aquifers, damage to the hydrocarbon reservoir, and loss of well profitability.
The job of cementing the casing in place has several potential pitfalls. For example, as the borehole wall can be quite irregular, the volume of the annular space between the casing and the borehole wall is somewhat unpredictable. Moreover, there may be voids, fractures, and/or porous formations that allow cement slurry to escape from the borehole. Conversely, fluids (including gasses) can become trapped and unable to quickly escape from the annular space, thereby preventing the cement slurry from fully displacing such materials from the annular space. (Any such undisplaced fluids provide potential fluid flow paths that can lead to a loss of zonal isolation). Accordingly, the cementing crew may have difficulty predicting how much of the well will be successfully cemented by a given volume of cement slurry.
Further, the chemical composition of the cement slurry may be altered for various reasons including slowing the setting time (i.e., the slurry's transition time from liquid to solid state). Depending on the downhole conditions, the expected setting time can be very different from the actual setting time. For example, temperature is a key factor for the setting time. Although temperature modeling software is available, there are many drivers that affect the downhole temperature during the cement curing process including: temperature of the injected cement slurry; temperature profile and heat conductivity of the formation; and heat of hydration. Consequently the actual temperature regime in the borehole may be different from the estimated profile and therefore the setting time may be different. Currently well operators and regulatory authorities rely upon “rules of thumb” and cement slurry tests undertaken with estimated parameters to govern how and when well operations may commence after cement placement. As incompletely set cement is more susceptible to damage, the uncertainty regarding setting time often requires a balancing of risks, e.g., balancing the risk of fractured cement with undesirable delays in completing the well. Moreover, unexpectedly lengthened setting times increase the risks of fluid influx from the formation, which can create undesired fluid flow paths.